1. Field of the Invention
The present invention relates to an ink-jet printing head and an ink-jet printing apparatus and method.
2. Description of the Related Art
Ink-jet printing apparatuses, which eject ink droplets from an ink-jet printing head to print an image, can print an image with various gradations by varying the size of the droplets.
A conventional ink-jet printing head, for example, the one described in U.S. Pat. No. 5,208,605, has two nozzle lines formed therein parallel with each other and extending in a direction crossing a scanning direction. One of the nozzle lines has a plurality of larger nozzles (larger ejection openings) arranged at equal intervals and through which large ink-droplets are ejected. In contrast, the other nozzle line has a plurality of smaller nozzles (smaller ejection openings) arranged at equal intervals and through which small ink-droplets are ejected. The larger and smaller nozzles are communication with a common ink supply port, and the same type of ink is ejected through these nozzles.
A printing head constructed in this manner ejects ink droplets through the larger and smaller nozzles while moving in the scanning direction, to form large and small ink dots on a printing medium.
Applicant of the present invention examined a printed image printed by using the conventional printing head. The printed image is obtained by high-density pixels and low-density pixels, high-density pixels corresponding to the large ink dots are formed by large ink-droplets ejected from the larger nozzles, and low-density pixels corresponding to the small ink dots are ejected from the smaller nozzles. Result of the examination, unwanted stripes and noticeable granularity are appeared in printed images, thereby making it difficult to print photograph-grade images.
Applicant of the present invention found out one of the causes of the phenomenon. It is caused by the arrangement in which the positions of the larger nozzles deviate from the position of the smaller nozzles in the direction of the nozzle lines. That is, in the conventional printing head, the position of large dots formed by large ink-dots ejected from the larger nozzles deviate from the position of small dots formed by small ink-dots ejected from the smaller nozzles, because the positions of the larger nozzles deviate from the position of the smaller nozzles in the direction of the nozzle lines. Therefore, if the large dot is formed on a center of a pixel, the position of the small dot deviate from a center of a low-density pixel formed by one small dot. Thus, in a low-density pixel in which a small dot is formed, a large and unwanted stripe shape blank is created within the pixel, the blank amounting to the deviation of the position of the small dot from the center.
Further, when printing an image is performed by using the conventional printing head, as shown in FIG. 20, the junction part between large dots D1 coincides with the junction part between small dots D3 on a line L extending along the scanning direction of the printing head. Accordingly, when the ejecting directions of the large ink-droplets and small ink-droplets are deviated each other, gap caused at the position corresponding to the junction part between large dots D1 links with gap caused at the position corresponding to the junction part between small dots D3. The linking gaps are appeared in the printed images, as the unwanted stripes.
Furthermore, as sown in FIG. 21, if the small dots D3 will be formed at the portion adjoined large dots D1, it is difficult to form the small dots D3 independently without overlapping with the large dots D1. The large dots D1 and the small dots D3 are into dots D0 larger than large dots D1. Therefore, noticeable granularity is appeared in printed images due to the dots D0.